A two-stroke engine of this kind is disclosed in international patent publication WO98/17901 and includes a combustion chamber defined by a cylinder and delimited by a reciprocating piston. The crankcase is connected to the combustion chamber via transfer channels. The first end of a transfer channel faces toward the cylinder and opens into the combustion chamber via an entry window lying in the cylinder wall and the lower second end of the transfer channel opens to the crankcase. The entry window of the transfer channel, which lies in the cylinder wall, is controlled by the piston in the manner of a slot control, that is, the entry window is opened or closed in dependence upon the stroke position of the piston.
The air/fuel mixture, which is necessary to operate the engine, is drawn in by suction through a mixture-preparation device and an inlet into the crankcase and, with a downward travel of the piston, is pushed into the combustion chamber via the transfer channels. To reduce the exhaust-gas emissions, fuel-free gas, especially air, is provided in the transfer channels arranged to the right and to the left of the outlet. This fuel-free gas is supplied to the transfer channels via respective gas channels.
In the induction stroke, and with the piston traveling upwards in the direction of top dead center, a mixture is drawn by suction into the crankcase, on the one hand, via the inlet from the mixture-preparation device; on the other hand, fuel-free air flows into the crankcase via the transfer channels from the gas channel. With the downwards travel of the piston in the direction of bottom dead center, the mixture is displaced from the crankcase via the transfer channels into the combustion chamber. For an operation as a scavenging engine, first, because of the charge of the transfer channels with air, fuel-free air flows into the combustion chamber ahead of the air/fuel mixture whereby the scavenging losses are reduced. In a subsequent upward stroke, residual amounts of the air/fuel mixture remain in the transfer channel from the previous stroke. These residual amounts are scavenged with fuel-free gas, especially air, in a next induction stroke. In practice, it has been shown that the inflowing gas flow of fuel-free air cannot always ensure a complete scavenging of the transfer channel so that residual amounts of the air/fuel mixture of a previous stroke enter the combustion chamber in a subsequent stroke together with the fuel-free air. For this reason, the scavenging losses increase. Because of the incomplete scavenging of the transfer channels with the fuel-free gas, the desired low exhaust-gas emissions often cannot be maintained.